kqueue: add EV_KEEPUDATA flag

[freebsdphab-AX9_cmx.ietfng.org]

Summary

This allows us to, for example, NOTE_TRIGGER or EV_{EN,DIS}ABLE events without overwriting the stashed pointer.

Test Plan

Manual verification of EV_KEEPUDATA behavior, new test functions

[domagoj.stolfa_gmail.com]

I've been using this locally and found it *extremely* useful, and frankly was quite surprised when I found out that EV_ENABLE/EV_DISABLE trashed udata originally.

[jhb]

I think the only thing that would be nice to add in addition are some tests for this. Probably best for that to add a new "native" test binary in tests/sys/kqueue instead of shoe-horning this into the NetBSD tests.

[domagoj.stolfa_gmail.com]

@jhb Any chance you could push this? I've been using this for months and it's worked well for me.